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  3. Passage Three (1) "We’ve been wondering what planet we’re first going

Passage Three (1) "We’ve been wondering what planet we’re first going

游客2024-11-03  31
问题     Passage Three
    (1)   "We’ve been wondering what planet we’re first going to look for life on. Now we know. " Rory Barnes, of the University of Washington, puts it nicely. Proxima Centauri, the star closest to the sun, has a planet. That planet weighs not much more than Earth and is therefore presumably rocky. And it orbits within its parent star’s habitable zone—meaning that, given an atmosphere, its surface temperature is likely to permit liquid water.
    (2)   A prize discovery, then, for astrobiologists such as Dr. Barnes. The discoverers themselves are a transnational team of astronomers who have been using telescopes at the European Southern Observatory (ESO) in the Atacama desert, in Chile, for planet-hunting. They have inferred its existence from its effect on its parent star’s light, and their paper in Nature describes what they have been able to deduce about it.
    (3)  Proxima Centauri b, as it is known, probably weighs between 1.3 and three times as much as Earth and orbits its parent star once every 11 days. This puts its distance from Proxima Centauri itself at 7m kilometres, which is less than a twentieth of the distance between Earth and the sun. It can remain temperate in such a close orbit only because Proxima is a red dwarf, and thus much cooler than the sun. It is not the only Earth-sized extrasolar planet known to orbit in a star’s habitable zone. There are about a dozen others. But it is the closest to Earth—so close, at four light-years, that it is merely outrageous, not utterly absurd, to believe a spaceship (admittedly a tiny one) might be sent to visit it. Before this happens, though, it will be subjected to intense scrutiny from Earth itself.
    (4)  That scrutiny will probably be led by ESO. The data which led to Proxima Centauri b’s discovery came from the observatory’s 3.6 metre telescope at La Silla, in Chile. But ESO is also building a much bigger device, the 39-metre European Extremely Large Telescope (EELT), at another site in Chile. Since the late 2000s Markus Kasper of ESO has led a team which is designing a specialised planet-spotting instrument, the Exoplanet Imaging Camera and Spectrograph (EPICS), to fit on this telescope. Dr. Kasper’s camera has a price tag of 50m ( $ 56m), and there have always been questions about whether it is worth the money. But EPICS stands a better chance of producing actual pictures of Proxima Centauri b, and being able to analyse its atmosphere, than any other instrument in the world (or off it). Its future can now scarcely be in doubt.
    (5)  The problem for astronomers trying to catch a glimpse of Proxima Centauri b is that, though close to the Earth by interstellar standards, it is closer to its parent star by more or less every other standard short of that of walking down the road to the chemist. Seen from Earth, star and planet are 35 thousandths of an arc second apart (an arc second is a 3,600th of a degree). Producing a picture that separates the two objects thus requires a telescope with a resolution good enough to distinguish between the left and right headlights of an oncoming car in Denver from the distance of Berlin.
    (6)  Things get worse. Dim as it is, Proxima Centauriis still more than 10m times brighter than its planet is expected to be. It is as though one of those headlights in Denver was actually the open door to a furnace, while the other was a tea light. This is what makes the E-ELT and EPICS crucial. EPICS contains a coronagraph—a tiny shield that blocks out a star’s light so that adjacent planets can be seen. Unfortunately, a coronagraph reduces a telescope’s resolution, meaning you need an even bigger one to see the target in the first place. To observe Proxima Centauri b using a coronagraph, and doing so in the infrared wavelengths that are likely to provide the best information about its atmosphere, you need a telescope at least 20 metres across; 30 metres would be better.
    (7)  The exciting thing about the planet’s spectrum, however it is measured, is that it might reveal the water content and chemical composition of Proxima Centauri b’s atmosphere, if it has one. And that might, in turn, give a clue as to whether it harbours life. Life on Earth leaves a sign of its existence in the atmosphere, in the form of oxygen. This is produced by plants and it is such a reactive chemical that if their photosynthesis stopped it would disappear rapidly from the air. Free oxygen in Proxima Centauri b’s atmosphere would therefore get a lot of people excited—but possibly without justification, for there are ways to put oxygen into atmospheres abiotically. A stronger indicator of life would be finding both oxygen and molecules associated with biology that cannot long persist in its presence, and must thus be produced continuously.
    (8)  Another way to look for life on Proxima Centauri b would be to search for radio signals. Life in general does not generate radiation at radio frequencies. But intelligent life does—at least it does on Earth. And that Earth-bound life also puts a tiny bit of effort into looking for such emissions from elsewhere, an endeavour known as the search for extraterrestrial intelligence, or SETI. There have been SETI studies of Proxima Centauri in past decades, but they have not been particularly sensitive.
    (9)  That there is intelligent life in the nearest planetary system to Earth’s is surely the longest of shots. And despite its nice-sounding location in the "habitable zone" , the presence of any sort of life on Proxima Centauri b is far from a foregone conclusion. For one thing, there are doubts about how easy it is for planets around red dwarfs to develop and retain atmospheres. Though such stars are cool for most of their existences, in their early years they burn bright. A planet close enough to one to stay warm in later life might have seen its atmosphere burned off in the star’s brief blazing youth. Even if it avoids this problem, it will still be whipped by the star’s magnetic field and lashed by its flares. Though they are dim, red dwarfs are given to all sorts of eruptive activity and pump out X-rays at a prodigious rate. These are both things which might make an atmosphere hard to hold on  to and life itself a bit tricky. [br] What do we know about Proxima Centauri b?
选项 A、There is atmosphere and water on it.
B、Some of its traits are similar to those of the Earth.
C、It is much closer to the sun than to the Earth.
D、It is very likely to find intelligent life on it.
答案 B
解析 细节题.文章第三段较为集中地介绍了行星Proxima Centauri b的具体情况，第一句介绍了它的大小、重量和运行规律，第二、三句将它与本星系恒星的距离和地球与太阳的距离进行比较，并指出它虽与恒星距离近，但是恒星温度不高，因此它也能保持适宜温度。第四句指出它是与地球大小近似的行星，且像地球一样有规律地围绕恒星运行，故答案为[B]。文章第一段提到，根据该行星的地表温度，如果它外层有大气，则很有可能存在液态水，但这都是推测，并没有得到证实，故排除[A]；文章并没有比较它与地球和与太阳之间的距离，因此[C]没有依据，应排除；对于这颗行星是否具备大气、液态水等孕育生命的基本条件，文章只提到一些推论，并没有暗示有较大的可能，故排除[D]。
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